The invention relates to a method and apparatus for recognizing a standstill condition when restarting a power converter-fed AC motor without a tachometer generator.
Downtimes can occur in manufacturing operations when a power converter which drives a motor cannot be immediately restarted (e.g., after a brief power failure or after having been turned off) because the motor has not yet reached a standstill condition. If a restart is attempted when the power converter frequency does not match the motor frequency of an asynchronous machine, no flux can build up after the restart operation. In a closed-loop control operation, for example, this can lead to generation of very large current amplitudes, causing the power converter to again shutdown due to an overcurrent status. An especially critical situation results when a power converter is automatically restarted without first ensuring the motor is at a standstill.
As soon as the operating voltage regains its previous level and the power converter is restarted, the increasing current flowing through the motor causes the difference between the output frequency of the converter assembly and the motor speed to become increasingly larger. For this reason, the power converter must be provided with a large power capacity, which is uneconomical.
Similarly, if the operating voltage has been removed from the motor for a relatively long period of time, the motor will likely have slowed to a relatively low speed. Immediate application of an operating voltage at the same frequency and the same level as before the failure will then cause a large difference between the output frequency of the power converter and the motor speed, thereby resulting in an undesirably large motor starting current. This phenomenon also occurs when a motor is deactivated due to an operating voltage failure, so that the motor then rotates in a free-running state when the operating voltage is again supplied to the motor at the same frequency and the same level as before the failure.
A well-known method for avoiding such a large motor starting current is to restart the motor only after it has come to a complete standstill regardless of the duration of the power failure. The stationary motor is then restarted by gradually increasing the frequency of the operating voltage supplied to the motor, which is why it takes a relatively long time to restart the motor and return to a steady operating condition. To overcome these disadvantages, commercial converter assemblies are sometimes equipped with a "SEARCH" option and/or a "RECOVERY" option.
The "RECOVERY" option uses a recovery circuit to rebuild the rotor flux which decays after a power converter is shutdown while the machine is running. Such decay occurs in accordance with the rotor time constants in response to an e-function. No large accelerating or braking torques may build-up during this flux build-up, which continues for a defined period of time, after which the closed-loop or open-loop controlled operation of the asynchronous machine is resumed. In the case of speed-controlled drives (i.e., drives whose rotational speed is subject to closed-loop control), the motor is to be restored to its original nominal speed after the recovery operation. The recovery circuit is also supposed to be used when the motor is not equipped with a tachometer.
Methods and apparatus for restarting other types of motors are known in the art. For example, German Patent 32 02 906 C2 discloses a method and apparatus for restarting an induction motor. This method involves supplying the motor with a search voltage, which voltage is itself insufficient to drive the motor, while at the same time varying the frequency of the power converter associated with the motor. While the frequency is being varied, the current flowing through the free-running motor is measured to determine the frequency of the detection voltage which matches the rotational speed of the free-running motor. The frequency so-determined is defined as the starting frequency. Another voltage, again insufficient to drive the motor but now conducted at the starting frequency, is then supplied to the motor. While retaining the starting frequency, this voltage is gradually increased from a starting voltage, and the frequency and the voltage are raised to predetermined values so that the motor operates normally. The method thereby avoids a large motor starting current that increases as the difference between the output frequency of the power converter and the motor speed increases.
German Laid-Open Print 35 43 983 A1 discloses a method for connecting a power converter to a still rotating, not-energized polyphase machine using a search operation that involves continually impressing a nominal current on the stator winding of the power converter, causing the power converter to pass through its possible frequency range. When the power converter frequency and the rotational speed of the rotor roughly conform, the stator voltage rises because of the resulting flux build-up. To this end, the magnitude of the stator voltage rise is continually monitor until a predetermined level is reached, at which point the search speed is reduced. A slip frequency window operation is then run using a diminished search speed. As a result, a flux builds up in the polyphase machine. A desired instantaneous working point of the polyphase machine is deemed to have been reached when this flux reaches a predetermined value. The search operation terminates, and the converter can be connected along with its control system to the still-rotating polyphase machine. This method thus enables the speed of the search operation to be considerably increased.
German Laid-Open Print 35 43 941 A1 discloses another method for determining the rotational speed of a still-rotating polyphase machine. This method involves evaluating at least one sinusoidal voltage that is induced by the remanence of the rotor in the stator windings, which voltage corresponds in frequency to the rotational speed of the rotor. This sinusoidal voltage, which is supplied to a stator terminal, is then converted by means of a square-wave shaper into a square-wave voltage having a symmetrical waveshape and a frequency and phase position conforming to the frequency and phase position of the induced sinusoidal voltage. An output signal is subsequently generated with a frequency proportional to the square-wave voltage. Since the frequency or cycle duration of this induced sinusoidal voltage is not dependent upon the machine type or the machine temperature, one obtains a method that applies generally to polyphase machines. By defining the rotational speed of the still-rotating polyphase machine in this manner, it is possible to connect the power converter (having a frequency corresponding to the rotor frequency) to the still-rotating polyphase machine substantially faster than the method described above because the search speed is no longer dependent upon the frequency window of the polyphase machine.
Yet another method and apparatus for restarting an induction motor are disclosed in EP Patent Application 0 469 177 A1. In this method, at least two remanence frequency values are determined from remanence voltage values measured at different instants, from which an acceleration value of the "coasting" (i.e., rotating as a result of inertia) motor is determined. At the end of this acceleration measurement, an output frequency value is set to the last-determined remanence frequency value. The acceleration measurement is then followed by the recovery of the motor, which takes place during an adjustable excitation period. Power converter pulses are released at the start of this excitation period, upon which the remanence of the machine is immediately lost. Therefore, during the excitation period, starting from the set output frequency value of the power converter, the output frequency value is interpolated with the determined acceleration value, so that the output frequency value is corrected to the actual rotational-speed value of the coasting machine. Thus, the interpolation of the output frequency value of the power converter represents a simulation of the actual rotational-speed value of the "coasting" machine; that is, a measured-value equivalent parameter is determined. The machine flux is also built up during the excitation period by setting the output voltage value of the frequency converter to high. As soon as the adjusted excitation time has elapsed, the "recovery" state is abandoned and a switch is made to closed-loop control operation. In this case, starting out from the last determined interpolated output frequency value of the frequency converter, the rotational speed is again brought to the original rotational speed in accordance with an adjusted starting time.
In this method, torque production during the recovery process should be kept as small as possible, which is accomplished by the motor allowing, for example, 90% of its magnetization to decay. A de-excitation time, which begins with the switching off of the power converter, is therefore adjusted to 2.3 times the rotor time constant.
The rise between the first and the last determined remanence frequency value is calculated during the adjustable acceleration measuring time. The shorter the measuring time, the faster the machine's current rotational speed is determined. The acceleration measuring time is converted into a number of periods of the remanent voltage, so that the actual acceleration measuring time changes slightly along with the changing rotational speed. The acceleration measuring time is supposed to correspond to at least the time of two periods of the remanent rotational speed.
The duration of the flux build-up can be determined using the adjustable excitation period. The longer the adjustable excitation period is, the more the recovery circuit works free from torque. By contrast, the shorter the excitation period is, the faster the current rise and the transition to closed-loop control operation occurs.
A disadvantage of this known method is that the rotational speed of a still-rotating polyphase machine cannot be determined without considerable expenditure of time and energy when the rotational speed is very low. At very low rotational speeds, the search operation is often interrupted after a pair of cycles with the result "machine not found." This requires that an operator then shut-down and restart the machine, which takes considerable time.
Moreover, at very low frequencies of the drive, the stator and lead resistances influence the voltage specifications, so that the "search" or the "recovery" of the polyphase machine are adversely affected. The result is that the "SEARCH" or "RECOVER" options can no longer be employed below a certain rotational speed with causing the power converter to malfunction.